Fiber-reinforced composite materials (or FRP) are widely used in many applications such as various industries, sporting goods and equipment for leisure time amusement as well as space and aviation fields. Owing to its properties characterized by light weight, high strength, and high elastic modulus, a carbon fiber-reinforced material (or CFRP) is used in a large amount for parts, which need to be light and highly rigid, among fiber-reinforced composite materials made by using fibers such as glass fibers, aramid fibers, boron fibers, and the like. For example, in industrial fields such as printing machines and film making machines, increase in productivity has been made possible by the use of a composite roll which is prepared by metal- or resin-plating the surface of a CFRP core and whose weight is 20 to 40% less than that of a conventional aluminum alloy roll.
Excellent strength and impact resistance are indispensable for a fiber-reinforced material. In the field of automotive parts, use of FRP for truck bodies and use of CFRP for leaf springs have been hither promoted. In these applications, in order to protect the persons in the vehicle in case an accident should happen, it is very important to impart excellent strength and impact resistance to the structural members of bodies and it is also very important to impart excellent impact resistance to the leaf springs and others which are subjected to the repetition of impact load in order to ensure long-pending reliability of automobiles. Conventionally, automotive parts have been made lighter by use of the above-described FRP in comparison with automotive parts made of metal. However, automotive parts using FRP cannot be said to be satisfactorily light because these automotive parts made of FRP need to maintain the same strength and impact resistance as those of automotive parts made of metal.
Moreover, lowness of elastic modulus, namely excellence in flexibility, is also one of the very important properties of a fiber-reinforced composite material depending on applications. In particular, the flexibility of sporting goods is said to exert a significant influence on the feeling of the players. For example, a golf club using a flexible shaft is said to be beneficial to beginners and female golfers. This is because, although their swing speed is slow, the use of such a club enables them to increase the head speed of the club so that a longer flying distance of ball can be expected due to the pliant suppleness of the shaft. Similarly, flexible ski boards are said to be suitable to beginners or women playing golf because turns utilizing the flexibility of boards become possible.
Conventionally, in order to obtain the above-mentioned shaped articles having low rigidity, glass fibers have often been used. However, the use of glass fibers is associated with disadvantages. For example, since the density of glass fiber is larger than that of carbon fiber or the like, the use of glass fiber brings about increase in weight of the shaped articles. Further, since the compressive strength of glass fiber is low despite its high tensile strength, a shaped article made of a glass fiber-reinforced composite material does not exhibit sufficient strength. Furthermore, in the aspect of vibration characteristics, since the vibration damping characteristics of glass fibers are inferior to those of carbon fibers, a shaped article made of a glass fiber-reinforced composite material exhibits a feeling inferior to that of a shaped article made of a carbon fiber-reinforced composite material.
Meanwhile, when designing FRP, it must be taken into account that FRP has anisotropy. That is, although high strength and elastic modulus are exhibited in the direction of orientation of the reinforcing fibers, both of strength and elastic modulus are extremely low in the direction at a right angle to the direction of orientation of the reinforcing fibers because the tensile strength of the matrix resin and the bonding strength in the interface between the matrix resin and the reinforcing fibers dominate the characteristics in that direction.
Further, caution must be exercised to the fact that delamination is liable to occur in FRP laminated materials. For example, in the case of FRP used for aircraft, a flying body such as a bird may collide with the body of the aircraft during flight or otherwise a member of maintenance staff may inadvertently drop a tool such as wrench, spanner, or the like on the body of the aircraft during maintenance service on the ground. These impacts cause serious delamination inside the FRP laminated materials and a significant reduction in mechanical properties, in particular compressive strength, thus presenting a big problem.
On the other hand, since repeated impact takes place in leaf springs of automobiles, it is very important to increase interlayer fracture toughness. As to structural members of bodies, it is also important to inhibit the delamination in order not to aggravate the mechanical properties by impact caused by, for example, an accident.
Heretofore, various methods have been employed to increase interlayer fracture toughness of FRP by making the matrix resin more tenacious or by using thermoplastic resins particles or short fibers between the layers of an FRP laminated material. However, these methods have been necessarily associated with disadvantages such as increase of process steps.